Organic electroluminescent element

ABSTRACT

An organic EL device includes a pair of electrodes and an organic compound layer between pair of electrodes. The organic compound layer includes an emitting layer including a first material, a second material and a third material, in which singlet energy EgS(H) of the first material, singlet energy EgS(H2) of the second material, and singlet energy EgS(D) of the third material satisfy a specific relationship.

TECHNICAL FIELD

The present invention relates to an organic electroluminescence device.

BACKGROUND ART

When a voltage is applied to an organic electroluminescence device(hereinafter referred to as an organic EL device), holes are injectedfrom an anode into an emitting layer and electrons are injected from acathode into the emitting layer. The injected holes and electrons arerecombined in the emitting layer to form excitons. Here, according tothe electron spin statistics theory, singlet excitons and tripletexcitons are generated at a ratio of 25%:75%. In the classificationaccording to the emission principle, in a fluorescent EL device whichuses emission caused by singlet excitons, an internal quantum efficiencyof the organic EL device is believed to be limited to 25%. On the otherhand, it has been known that the internal quantum efficiency can beimproved up to 100% under efficient intersystem crossing from thesinglet excitons in a phosphorescent EL device which uses emissioncaused by triplet excitons.

A technology for extending a lifetime of a fluorescent organic EL devicehas recently been improved and applied to a full-color display of amobile phone, TV and the like. However, an efficiency of the fluorescentorganic EL device is required to be improved.

Based on such a background, a highly efficient fluorescent organic ELdevice using delayed fluorescence has been proposed and developed. Forinstance, Patent Literatures 1 and 2 each disclose an organic EL deviceusing TTF (Triplet-Triplet Fusion) mechanism that is one of mechanismsfor delayed fluorescence. The TTF mechanism utilizes a phenomenon inwhich a singlet exciton is generated by collision between two tripletexcitons.

By using delayed fluorescence caused by the TTF mechanism, it isconsidered that an internal quantum efficiency can be theoreticallyraised up to 40% even in fluorescent emission. However, as compared withphosphorescent emission, the fluorescent emission still needs to beimproved in efficiency. Accordingly, in order to further improve theinternal quantum efficiency, an organic EL device using another delayedfluorescence mechanism has been studied.

For instance, TADF (Thermally Activated Delayed Fluorescence) mechanismis used. The TADF mechanism utilizes a phenomenon in which reverseintersystem crossing from triplet excitons to singlet excitons isgenerated by using a material having a small energy difference (ΔST)between the singlet level and the triplet level. An organic EL deviceusing the TADF mechanism is disclosed in, for instance, non-PatentLiterature 1. In the organic EL device of non-Patent Literature 1, acompound having a small ΔST is used as a dopant material to causereverse intersystem crossing from the triplet level to the singlet levelby heat energy. It is considered that the internal quantum efficiencycan be theoretically raised up to 100% even in fluorescent emission byusing delayed fluorescence by the TADF mechanism,

Non-Patent Literature 2 discloses an organic EL device having a dopedfilm including a specific host material and a specific compound having aSpiro skeleton as a dopant material. The organic EL device exhibits ahigh external quantum efficiency by using the TADF mechanism.

CITATION LIST Patent Literature(s)

-   Patent Literature 1: International Publication No. WO2010/134350-   Patent Literature 2: International Publication No. WO2011/070963

NON-PATENT LITERATURE(S)

-   Non-Patent Literature 1: “Expression of Highly-Efficient    Thermally-Activated Delayed-Fluorescence and Application thereof to    OLED” Organic EL Symposium, proceeding for the tenth meeting edited    by Chihaya Adachi et al., pp. 11-12, Jun. 17-18, 2010-   Non-Patent Literature 2: “Development of Highly Efficient    Electroluminescence Devices Utilizing Thermally Activated Delayed    Fluorescence of Spiro-Structured Molecules” Chemical Society of    Japan, proceeding for the 92nd Spring meeting on Mar. 25-28, 2012    edited by Chihaya Adachi et al., 3M3-37

SUMMARY OF THE INVENTION Problem(s) to be Solved by the Invention

Although the organic EL devices disclosed in the non-Patent Literatures1 and 2 exhibit the maximum luminous efficiency in a low current densityrange at 0.01 mA/cm², so-called roll-off is generated in a practicallyhigh current density range ranging from approximately 1 mA/cm² to 10mA/cm², so that the luminous efficiency is decreased.

Accordingly, it is considered that many practical problems in usingdelayed fluorescence by the TADF mechanism are left unsolved, amongwhich improvement in the luminous efficiency in the practically highcurrent density range has been particularly demanded.

An object of the invention is to provide an organic electroluminescencedevice efficiently emitting light in a practically high current densityrange.

Means for Solving the Problem(s)

According to an aspect of the invention, an organic electroluminescencedevice includes a pair of electrodes and an organic compound layertherebetween, the organic compound layer including an emitting layerincluding: a first material; a second material; and a third material, inwhich singlet energy EgS(H1) of the first material, singlet energyEgS(H2) of the second material, and singlet energy EgS(D) of the thirdmaterial satisfy a relationship of numerical formulae (1) and (2) below,a difference ΔST(H1) between the singlet energy EgS(H1) of the firstmaterial and an energy gap Eg_(77K)(H1) at 77K of the first materialsatisfies a relationship of a numerical formula (3) below, the secondhost material is a compound having a fused aromatic hydrocarbon grouphaving 10 to 30 ring carbon atoms or a fused aromatic heterocyclic grouphaving 8 to 30 ring atoms, and the third material is a fluorescentmaterial.

EgS(H1)>EgS(D)  (1)

EgS(H2)>EgS(D)  (2)

ΔST(H1)=EgS(H1)−Eg _(77K)(H1)<0.3 [eV]  (3)

BRIEF DESCRIPTION OF DRAWING(S)

FIG. 1 schematically shows an exemplary arrangement of an organicelectroluminescence device according to an exemplary embodiment of theinvention.

FIG. 2 shows an example of physics models with aggregate formation.

FIG. 3 shows a relationship between energy levels of a host material anda dopant material in an emitting layer.

FIG. 4 shows a relationship between energy levels of a host material anda dopant material in an emitting layer.

FIG. 5 shows a measurement system of transitional EL waves.

FIG. 6A shows a measurement method of a ratio of luminous intensitiesderived from delayed fluorescence and is a graph showing time-varyingluminous intensities of the EL device.

FIG. 6B shows a measurement method of a ratio of luminous intensitiesderived from delayed fluorescence and is a graph showing time-varyingreverse square root of the luminous intensities.

FIG. 7 shows a relationship between energy levels of a host material anda dopant material in an emitting layer.

FIG. 8 shows a relationship among energy levels of a first material, asecond material and a third material in the emitting layer.

FIG. 9 is a graph showing time-varying luminous intensities of theorganic EL device according to the exemplary embodiment of theinvention.

FIG. 10 is another graph showing time-varying luminous intensities ofthe organic EL device according to the exemplary embodiment of theinvention.

FIG. 11 is a graph showing a relationship between a current density andthe luminous efficiency of the organic EL device according to theexemplary embodiment of the invention.

FIG. 12 is another graph showing a relationship between a currentdensity and the luminous efficiency of the organic EL device accordingto the exemplary embodiment of the invention.

DESCRIPTION OF EMBODIMENT(S) Arrangement(s) of Organic EL Device

Arrangements) of an organic EL device according to an exemplaryembodiment of the invention will be described below.

The organic EL device according to the exemplary embodiment includes apair of electrodes and an organic compound layer therebetween. Theorganic compound layer includes at least one layer formed of an organiccompound. The organic compound layer may include an inorganic compound.

In the organic EL device according to the exemplary embodiment, at leastone layer of the organic compound layer includes an emitting layer.Accordingly, the organic compound layer may be provided by a singleemitting layer. Alternatively, the organic compound layer may beprovided by layers applied in a known organic EL device such as a holeinjecting layer, a hole transporting layer, an electron injecting layer,an electron transporting layer, a hole blocking layer and an electronblocking layer.

The following are representative structure examples of an organic ELdevice:

(a) anode/emitting layer/cathode;(b) anode/hole injecting⋅transporting layer/emitting layer/cathode;(c) anode/emitting layer/electron injecting⋅transporting layer/cathode;(d) anode/hole injecting⋅transporting layer/emitting layer/electroninjecting transporting layer/cathode; and(e) anode/hole injecting⋅transporting layer/emitting layer/blockinglayer/electron injecting⋅transporting layer/cathode.

While the arrangement (d) is preferably used among the abovearrangements, the arrangement of the invention is not limited to theabove arrangements.

It should be noted that the aforementioned “emitting layer” is anorganic compound layer generally employing a doping system and includinga first material, a second material and a third material. In general,the first and second materials promote recombination of electrons andholes and transmit excitation energy generated by recombination to thethird material. The first and second materials are often referred to asa host material. Accordingly, the first material is referred to as afirst host material and the second material is referred to as a secondhost material in descriptions hereinafter. In general, the thirdmaterial receives the excitation energy from the host material (thefirst and second materials) to exhibit a high luminescent performance.The third material is often referred to as a dopant material.Accordingly, the third material is referred to as a dopant material indescriptions hereinafter. The dopant material is preferably a compoundhaving a high quantum efficiency. In the exemplary embodiment, in atleast one emitting layer, a material emitting fluorescence (fluorescentdopant material) is used as the dopant material.

The “hole injecting/transporting layer” means “at least one of a holeinjecting layer and a hole transporting layer” while the “electroninjecting/transporting layer” means “at least one of an electroninjecting layer and an electron transporting layer.” Herein, when thehole injecting layer and the hole transporting layer are provided, thehole injecting layer is preferably close to the anode. When the electroninjecting layer and the electron transporting layer are provided, theelectron injecting layer is preferably close to the cathode.

In the exemplary embodiment, the electron transporting layer means anorganic layer having the highest electron mobility among organiclayer(s) providing an electron transporting zone existing between theemitting layer and the cathode. When the electron transporting zone isprovided by a single layer, the single layer is the electrontransporting layer. Moreover, a blocking layer having an electronmobility that is not always high may be provided as shown in thearrangement (e) between the emitting layer and the electron transportinglayer in order to prevent diffusion of excitation energy generated inthe emitting layer. Thus, the organic layer adjacent to the emittinglayer is not always an electron transporting layer.

FIG. 1 schematically shows an exemplary structure of the organic ELdevice according to the exemplary embodiment of the invention.

An organic EL device 1 includes a light-transmissive substrate 2, ananode 3, a cathode 4, and an organic compound layer 10 disposed betweenthe anode 3 and the cathode 4.

The organic compound layer 10 includes an emitting layer 5 containing ahost material and a dopant material. The organic compound layer 10 alsoincludes a hole transporting layer 6 between the emitting layer 5 andthe anode 3. The organic compound layer 10 further includes an electrontransporting layer 7 between the emitting layer 5 and the cathode 4.

Emitting Layer

In this exemplary embodiment, as described above, the first, second andthird materials satisfying specific conditions are respectively used asthe first host material, the second host material and the dopantmaterial in the emitting layer. The materials and specific conditionswill be described below.

In the emitting layer, singlet energy EgS(H1) of the first hostmaterial, singlet energy EgS(H2) of the second host material and singletenergy EgS(D) of the dopant material satisfy a relationship according tothe following numerical formulae (1) and (2), and a difference ΔST(H1)between the singlet energy EgS(H1) of the first host material and anenergy gap Eg_(77K)(H1) at 77K of the first host material satisfies arelationship according to the following numerical formula (3).

EgS(H1)>EgS(D)  (1)

EgS(H2)>EgS(D)  (2)

ΔST(H1)=EgS(H1)−Eg _(77K)(H1)<0.3 [eV]  (3)

It is preferable that the difference ΔST(H1) between the singlet energyEgS(H1) of the first material and the energy gap Eg_(77K)(H1) at 77K ofthe first material satisfies a relationship of the following numericalformula (4).

ΔST(H1)=EgS(H1)−Eg _(77K)(H1)<0.2 [eV]  (4)

It is preferable that an energy gap Eg_(77K)(H2) at 77K of the secondhost material and an energy gap Eg_(77K)(D) at 77K of the dopantmaterial satisfy a relationship of the following numerical formula (5).

Eg _(77K)(H2)<Eg _(77K)(D)  (5)

When the energy gap of the second host material and the energy gap ofthe dopant material satisfy the numerical formula (5), energy can betransferred from the triplet level of the dopant material to the tripletlevel of the second host material. With this energy transfer, tripletexcitons contributing to a below-described TTF mechanism are increasedin the second host material, so that energy is more efficientlytransferred.

It is preferable that the energy gap Eg_(77K)(H1) at 77K of the firsthost material and the energy gap Eg_(77K)(H2) at 77K of the secondmaterial satisfy a relationship of the following numerical formula (6).

Eg _(77K)(H1)−Eg _(77K)(H2)>0.5 [eV]  (6)

When the energy gap of the first host material and the energy gap of thesecond host material satisfy the numerical formula (6), the tripletexcitons of the first host material are unlikely to be transferred fromthe triplet level of the first host material to the triplet level of thesecond host material, so that energy transfer from the first hostmaterial can be inhibited.

The energy gap Eg_(77K)(H1) and the energy gap Eg_(77K)(H2) preferablysatisfy a relationship of the following numerical formula (6-1),particularly preferably a relationship of the following numericalformula (6-2).

Eg _(77K)(H1)−Eg _(77K)(H2)≥0.8 [eV]  (6-1)

Eg _(77K)(H1)−Eg _(77K)(H2)≥0.9 [eV]  (6-2)

The energy gap Eg_(77K)(H1) at 77K of the first host material and theenergy gap Eg_(77K)(D) at 77K of the dopant material satisfy arelationship of the following numerical formula (7).

ΔT=Eg _(77K)(H1)−Eg _(77K)(D)>0.5 [eV]  (7)

When the energy gap of the first host material and the energy gap of thedopant material satisfy the numerical formula (7), the excitons of thefirst host material are unlikely to be transferred from the tripletlevel of the first host material to the triplet level of the dopant hostmaterial, so that energy transfer from the first host material can beinhibited.

The energy gap Eg_(77K)(H1) and the energy gap Eg_(77K)(D) preferablysatisfy a relationship of the following numerical formula (7-1),particularly preferably a relationship of the following numericalformula (7-2).

ΔT=Eg _(77K)(H1)−Eg _(77K)(H2)≥0.9 [eV]  (7-1)

ΔT=Eg _(77K)(H1)−Eg _(77K)(H2)≥1.0 [eV]  (7-2)

A difference in the energy gap between the first host material and thefluorescent dopant material (Eg_(77K)(H1)−Eg_(77K)(D)) is denoted by ΔT.

First Host Material ΔST

The organic EL device emits light with a high efficiency in a highcurrent density range by using a compound having a small energydifference (ΔST) between singlet energy EgS and triplet energy EgT asthe first host material. The ΔST of the first host material is referredto as ΔST(H1).

From quantum chemical viewpoint, decrease in the difference (ΔST)between the singlet energy EgS and the triplet energy EgT can beachieved by a small exchange interaction therebetween. Physical detailsof the relationship between ΔST and the exchange interaction areexemplarily described in the following:

-   Literature 1: Organic EL Symposium, proceeding for the tenth meeting    edited by Chihaya Adachi et al., S2-5, pp. 11-12; and-   Literature 2: Organic Photochemical Reaction Theory edited by    Katsumi Tokumaru, Tokyo Kagaku Dojin Co., Ltd. (1973).

Such a material can be synthesized according to molecular design basedon quantum calculation. Specifically, the material is a compound inwhich a LUMO electron orbit and a HOMO electron orbit are localized toavoid overlapping.

Examples of the compound having a small ΔST, which is used as the firsthost material in the exemplary embodiment, are compounds in which adonor element is bonded to an acceptor element in a molecule and ΔST isin a range of 0 eV or more and less than 0.3 eV in terms ofelectrochemical stability (oxidation-reduction stability).

A more preferable compound is such a compound that dipoles formed in theexcited state of a molecule interact with each other to form anaggregate having a reduced exchange interaction energy. According toanalysis by the inventors, the dipoles are oriented substantially in thesame direction in the compound, so that ΔST can be further reduced bythe interaction of the molecules. In such a case, ΔST can be extremelysmall in a range of 0 eV to 0.2 eV.

Aggregate

Decrease in the energy difference (ΔST) between the singlet energy EgSand the triplet energy EgT can also be achieved by aggregate formation.Herein, the aggregate does not reflect an electronic state by a singlemolecule, but the aggregate is provided by several molecules physicallyapproaching each other. After the plurality of molecules approach eachother, electronic states of a plurality of molecules are mixed andchanged, thereby changing an energy level. A value of singlet energy ismainly decreased, thereby decreasing a value of ΔST. The decrease in thevalue of ΔST by the aggregate formation can also be explained by Davydovsplitting model showing that two molecules approach each other to changeelectronic states thereof (see FIG. 2). As shown in Davydov splittingmodel, it is considered that change of the electronic states by twomolecules different from change of an electronic state by a singlemolecule is brought about by two molecules physically approaching eachother. A singlet state exists in two states represented by S1-m ⁺ andS1-m ⁻. A triplet state exists in two states represented by T1-m ⁺ andT1-m ⁻. Since S1-m ⁻ and T1-m ⁻ showing a lower energy level exist, ΔSTrepresenting a difference between S1-m ⁻ and T1-m ⁻ becomes smaller thanthat in the electronic state by a single molecule.

The Davydov splitting model is exemplarily described in the following:

-   Literature 3: J. Kang, et al, International Journal of Polymer    Science, Volume 2010, Article ID 264781;-   Literature 4: M. Kasha, et al, Pure and Applied Chemistry, Vol. 11,    pp 371, 1965; and-   Literature 5: S. Das, et al, J. Phys. Chem. B. vol. 103, p 209,    1999.

The inventors found usage of sublevels of a singlet state and a tripletstate of a compound easily forming an aggregate in a thin film, andconsequent possibility of promotion of reverse intersystem crossing bymolecules and aggregates in the thin film.

For instance, a compound having a large half bandwidth of aphotoluminescence spectrum is considered to easily form an aggregate ina thin film of the compound. A relationship between the half bandwidthof the photoluminescence spectrum and easy formability of the aggregatecan be estimated as follows.

In a compound having a property of typically existing as a singlemolecule without forming an aggregate, a vibrational level is lessrecognized in the singlet state, so that a narrow half bandwidth of thephotoluminescence spectrum is observed. For instance, CBP(4,4′-bis[9-dicarbazolyl]-2,2′-biphenyl) exhibits a property totypically exist as a single molecule, in which a half bandwidth of aphotoluminescence spectrum is about 50 nm.

On the other hand, in the compound easily forming the aggregate, aplurality of molecules electronically influence each other, whereby alot of vibrational levels exist in the singlet state. As a result, sincethe vibrational levels of the singlet state are often relaxed to theground state, the half bandwidth of the photoluminescence spectrum isincreased.

Such a compound easily forming the aggregate is expected to have a lotof vibrational levels even in a triplet state. Consequently, it isspeculated that ΔST in relation to heat is decreased through thesublevels to promote the reverse intersystem crossing, since a lot ofsublevels exist between the singlet state and the triplet state.

It should be noted that the aggregate according to the exemplaryembodiment means that a single molecule forms any aggregate with anothersingle molecule. In other words, a specific aggregate state is not shownin the exemplary embodiment. An aggregate state of an organic moleculeis probably formable in various states in a thin film, which isdifferent from an aggregate state of an inorganic molecule.

TADF Mechanism

As described above, when ΔST(H1) of the organic material is small,reverse intersystem crossing from the triplet level of the first hostmaterial to the singlet level of the first host material is easilycaused by heat energy given from the outside. Herein, an energy stateconversion mechanism to perform spin exchange from the triplet state ofelectrically excited excitons within the organic EL device to thesinglet state by reverse intersystem crossing is referred to as TADFMechanism.

In the exemplary embodiment, since the compound having a small ΔST(H1)is used as the first host material, reverse intersystem crossing fromthe triplet level of the first host material to the singlet level of thefirst host material is easily caused by heat energy given from theoutside.

FIG. 3 shows a relationship in energy level between the first hostmaterial and the dopant material in the emitting layer. In FIG. 3, S0represents a ground state, S1 _(H1) represents a lowest singlet state ofthe first host material, T1 _(H1) represents a lowest triplet state ofthe first host material, S1 _(D) represents a lowest singlet state ofthe dopant material, and T1 _(D) represents a lowest triplet state ofthe dopant material. As shown in FIG. 3, a difference between S1 _(H1)and T1 _(H1) corresponds to ΔST(H1), a difference between S1 _(H1) andS0 corresponds to EgS(H1), a difference between S1 _(D) and S0corresponds to EgS(D), and a difference between T1 _(H1) and T1 _(D)corresponds to ΔT. A dotted-line arrow shows energy transfer between therespective excited states in FIG. 3.

As described above, a compound having a small ΔST(H) is selected as thecompound for the first host material in the exemplary embodiment. Thisis because the material having a small ΔST(H1) is considered to easilycause reverse intersystem crossing from the triplet excitons generatedin the lowest triplet state T1 _(H1) to the lowest singlet state S1_(H1) of the first host material by heat energy. Due to the smallΔST(H1), reverse intersystem crossing is easily caused, for instance,even around a room temperature. When the reverse intersystem crossing isthus easily caused, a ratio of energy transfer from the first hostmaterial to the lowest singlet state S1 _(D) of the fluorescent dopantmaterial is increased by Førster transfer, resulting in improvement in aluminous efficiency of a fluorescent organic EL device.

In other words, use of the compound having a small ΔST(H1) as the firsthost material increases emission by the TADF mechanism, so that adelayed fluorescence ratio becomes large. When the delayed fluorescenceratio is large, a high internal quantum efficiency is achievable. It isconsidered that the internal quantum efficiency can be theoreticallyraised up to 100% even by using delayed fluorescence by the TADFmechanism.

FIG. 4 shows a relationship in energy level between the first hostmaterial and the dopant material in the emitting layer in the TADFmechanism described in Patent Literature 1. In FIG. 4, S0, S1 _(H1), T1_(H1), S1 _(D), and T1 _(D) represent the same as those in FIG. 3. Adotted-line arrow shows energy transfer between the respective excitedstates. As shown in FIG. 4, a material having a small ΔST(D) is used asthe dopant material in the TADF mechanism described in non-PatentLiterature 1. Accordingly, energy is transferred from the lowest tripletstate T1 _(H1) of the first host material to the lowest singlet state S1_(D) or the lowest triplet state T1 _(D) of the dopant material byDexter transfer. Further, reverse intersystem crossing from the lowesttriplet state T1 _(D) to the lowest singlet state S1 _(D) of the dopantmaterial is possible by heat energy. As a result, fluorescent emissionfrom the lowest singlet state S1 _(D) of the dopant material can beobserved. It is considered that the internal quantum efficiency can betheoretically raised up to 100% also by using delayed fluorescence bythe TADF mechanism.

The inventors herein employ a fluorescent compound having a smallΔST(H1) as described in non-Patent Literature 1 as the first hostmaterial in a host-dopant system. The reasons are detailed as follows.

First, considering conversion of energy states on the dopant material bythe TADF mechanism, the dopant material has a relatively high singletenergy for fluorescent emission and triplet energy approximatelyequivalent to the singlet energy. In order to efficiently trap thetriplet energy within the emitting layer, it is necessary to select thefirst host material having larger triplet energy. If a typical organicmaterial usually having a large ΔST is used as the first host material,the singlet energy of the first host material, i.e., an energy gapbetween a HOMO level and a LUMO level becomes extremely large. As aresult, an energy gap between the first host material and a carriertransporting layer adjacent to the emitting layer becomes large, so thatinjection of carriers to the emitting layer is considered to becomedifficult. Accordingly, the inventors consider that conversion of theenergy states by the TADF mechanism is preferably performed on the firsthost material, whereby the carriers are advantageously injected to theemitting layer and are easily balanced in the entire organic EL device.

Secondly, the inventors believe it possible to suppress decrease in aluminous efficiency caused by Triplet-Triplet-Annihilation in a highcurrent density range by using the fluorescent compound having a smallΔST(H1) as the first host material. Herein, Triplet-Triplet-Annihilation(hereinafter, referred to as TTA) is a physical phenomenon in whichlong-life triplet excitons generated on a molecule are adjacent to eachother at a high density to collide with each other and are thermallydeactivated.

The inventors believe it possible to suppress decrease in the luminousefficiency in the high current density range to some extent in thehost-dopant system in which the triplet energy is difficult to transitfrom the first host material to the dopant material. In the exemplaryembodiment, the compound having a small ΔST is used as the first hostmaterial of the emitting layer. After reverse intersystem crossing froma triplet excited level of the first host material to a singlet excitedlevel thereof by the TADF mechanism, energy is transferred to a singletexcited level of the dopant material. Accordingly, the generated tripletexcitons are kept in a triplet excited state on the first host materialwhose abundance ratio is high in the emitting layer. On the other hand,if the compound having a small ΔST is used as the dopant material in theemitting layer, the generated triplet excitons are kept in a tripletexcited state on the dopant material whose abundance ratio is extremelylow in the emitting layer. In other words, the inventors believe itpreferable to design a system that avoids concentration of tripletexcited state on the dopant material in driving the organic EL device inthe high current density range. Accordingly, in the exemplaryembodiment, the inventors employ the compound having a small ΔST(H1) asthe first host material.

Thirdly, a material having a high emission quantum efficiency can beeasily selected as the dopant material by using a material causingreverse intersystem crossing from the triplet level to the singlet levelas the first host material. As a result, emission of the singletexcitons is quickly relaxed after energy transfer thereof to the dopantmaterial, so that energy quenching in the high current density range issuppressible. In the host-dopant system in a fluorescent device,generally, the first host material has a carrier transporting functionand an exciton generating function and the dopant material has anemission function. This system is for separating the carriertransporting function and the emission function of the emitting layer.Accordingly, effective organic EL emission is promoted by doping a smallamount of a dopant material having a high emission quantum efficiencyinto the emitting layer. The emitting layer according to the exemplaryembodiment is required to have a function to cause reverse intersystemcrossing by the TADF function in addition to a typical function of theemitting layer. By requiring the first host material to have thefunction to cause reverse intersystem crossing by the TADF mechanism,the inventors increased options for the dopant material having a highemission quantum efficiency which largely contributes to the luminousefficiency of the organic EL device. With this arrangement, afluorescent dopant material typically known as being highly efficientcan be selected.

Relationship Between EgT and Eg_(77K)

In this exemplary embodiment, the compound having ΔST equal to or lessthan a predetermined value is used. The aforementioned triplet energyEgT is different from a typically defined triplet energy. Such adifference will be described below.

For general measurement of the triplet energy, a target compound to bemeasured is dissolved in a solvent to form a sample. A phosphorescentspectrum (ordinate axis: phosphorescent luminous intensity, abscissaaxis: wavelength) of the sample is measured at a low temperature (77K).A tangent is drawn to the rise of the phosphorescent spectrum on theshort-wavelength side. The triplet energy is calculated by apredetermined conversion equation based on a wavelength value at anintersection of the tangent and the abscissa axis.

As described above, the compound used for the first host material in theexemplary embodiment has a small ΔST. When ΔST is small, intersystemcrossing and reverse intersystem crossing are likely to occur even at alow temperature (77K), so that the singlet state and the triplet statecoexist. As a result, the spectrum to be measured in the same manner asthe above includes emission from both the singlet state and the tripletstate. Although it is difficult to distinguish emission from the singletstate from emission from the triplet state, the value of the tripletenergy is basically considered dominant.

Accordingly, in order to distinguish the triplet energy EgT in theexemplary embodiment from the typical triplet energy EgT in a strictmeaning although the measurement method is the same, the triplet energyEgT in the exemplary embodiment is defined as follows. A target compoundto be measured is dissolved in a solvent to form a sample. Aphosphorescent spectrum (ordinate axis: phosphorescent luminousintensity, abscissa axis: wavelength) of the sample is measured at a lowtemperature (77K). A tangent is drawn to the rise of the phosphorescentspectrum on the short-wavelength side. Energy is calculated as an energygap Eg_(77K) by a predetermined conversion equation based on awavelength value at an intersection of the tangent and the abscissaaxis. ΔST is defined as a difference between the singlet energy EgS andthe energy gap Eg_(77K). Accordingly, ΔST(H1) is represented by theabove formula (3).

The triplet energy measured in a solution state may include an error byinteraction between the target molecule and the solvent. Accordingly, asan ideal condition, a measurement in a thin film state is desired inorder to avoid the interaction between the target molecule and thesolvent. In this exemplary embodiment, the molecule of the compound usedas the first host material exhibits a photoluminescence spectrum havinga broad half bandwidth in a solution state, which strongly impliesaggregate formation also in the solution state. Accordingly, thesolution state is considered to provide the same conditions as in a thinfilm state. Consequently, in this exemplary embodiment, a measurementvalue of the triplet energy in the solution state is used.

Singlet Energy EgS

The singlet energy EgS in the exemplary embodiment is defined based oncalculation by a typical method. Specifically, the target compound isevaporated on a quartz substrate to prepare a sample. An absorptionspectrum (ordinate axis: absorbance, abscissa axis: wavelength) of thesample is measured at a normal temperature (300K). A tangent is drawn tothe rise of the absorption spectrum on the long-wavelength side. Thesinglet energy EgS is calculated by a predetermined conversion equationbased on the tangent and the wavelength value at the intersection. EgSin aggregate formation corresponds to an energy gap between S1-m ⁻ andthe ground state S0 in the Davydov splitting model.

The calculation of the singlet energy EgS and the energy gap Eg_(77K)will be described in detail later.

Delayed Fluorescence Ratio

A delayed fluorescence ratio according to the organic EL device of theexemplary embodiment exceeds the theoretical upper-limit of a delayedfluorescence ratio (TTF ratio) of a case where it is assumed thatdelayed fluorescence is generated only by the TTF mechanism. In otherwords, according to the exemplary embodiment, an organic EL devicehaving a higher internal quantum efficiency is achievable.

The delayed fluorescence ratio is measurable by a transitional ELmethod. The transitional EL method is for measuring reduction behavior(transitional property) of EL emission after pulse voltage applied onthe device is removed. EL luminous intensity is classified into aluminescence component from singlet excitons generated in firstrecombination and a luminescence component from singlet excitonsgenerated through triplet excitons. Since lifetime of the singletexcitons generated in the first recombination is very short at anano-second order, EL emission from the singlet excitons generated inthe first recombination is rapidly reduced after removal of pulsevoltage.

On the other hand, since delayed fluorescence provides emission fromsinglet excitons generated through long-life triplet excitons, ELemission is gradually reduced. Thus, since there is a large differencein time between emission from the singlet excitons generated in thefirst recombination and emission from the singlet excitons derived fromthe triplet excitons, a luminous intensity derived from delayedfluorescence is obtainable. Specifically, the luminous intensity can bedetermined by the following method.

Transitional EL waveform is measured as follows (see FIG. 5). Pulsevoltage waveform outputted from a voltage pulse generator (PG) 11 isapplied on an organic EL device (EL) 12. The applied voltage waveform isloaded in an oscilloscope (OSC) 13. When pulse voltage is applied on theorganic EL device 12, the organic EL device 12 generates pulse emission.This emission is loaded in the oscilloscope (OSC) 13 through aphotomultiplier (PMT) 14. The voltage waveform and the pulse emissionare synchronized and loaded in a personal computer (PC) 15.

The ratio of luminous intensity derived from delayed fluorescence isdefined as follows based on analysis of the transitional EL waveform.

It should be noted that a formula to calculate a TTF ratio described inInternational Publication No. WO2010/134352 may be used for calculationof the ratio of luminous intensity derived from delayed fluorescence.

It is considered that a delayed fluorescence component defined in theexemplary embodiment includes thermally activated delayed fluorescence(TADF mechanism) described in the exemplary embodiment in addition tothe luminescence component derived from TTF. For this reason, in theexemplary embodiment, a ratio of the delayed fluorescence componentcalculated according to the following numerical formula (14) is referredto as a delayed fluorescence ratio, not as a TTF ratio.

The delayed fluorescence ratio is calculated according to the numericalformula (14).

$\begin{matrix}{{Numerical}\mspace{14mu} {Formula}\mspace{14mu} 1} & \; \\{\frac{1}{\sqrt{I}} \propto {A + {\gamma \cdot t}}} & (14)\end{matrix}$

In the numerical formula (14), I represents luminous intensity derivedfrom delayed fluorescence. A represents a constant. The measuredtransitional EL waveform data is fit in the numerical formula (14) toobtain the constant A. Here, a luminous intensity 1/A² at the time t=0when pulse voltage is removed is defined as the ratio of luminousintensity derived from delayed fluorescence.

A graph of FIG. 6(A) shows a measurement example where a predeterminedpulse voltage is applied on the organic EL device and then the pulsevoltage is removed and shows time-varying luminous intensities of theorganic EL device.

The pulse voltage was removed at the time of about 3×10⁻⁸ seconds in thegraph of FIG. 6 (A). In the graph of FIG. 6(A), the luminous intensitywhen the voltage is removed is defined as 1.

After rapid reduction before the elapse of about 2×10⁻⁷ seconds afterthe voltage removal, a gradual reduction component appears.

In the graph of FIG. 6(B), the voltage removal time is a starting pointand the reverse square root of luminous intensity before the elapse of1.5×10⁻⁵ seconds after voltage removal is plotted. Fitting is conductedas follows.

A value at an intersection A of the ordinate axis and the linear lineextended to the starting point is 1.55. Accordingly, the ratio ofluminous intensity derived from the delayed fluorescence obtained fromthe transitional EL waveform is 1/(1.55)²=0.41, which means 41% of theluminous intensity is derived from the delayed fluorescence. In otherwords, the ratio of luminous intensity exceeds 37.5%, i.e., the supposedtheoretical upper-limit of the TTF ratio.

The luminous intensity derived from the delayed fluorescence obtainedfrom the transitional EL waveform is variable in accordance withmeasurement temperatures. Such a phenomenon is considered to be inherentmostly in fluorescent emission by the TADF mechanism

The luminous intensity is preferably fitted in a linear line by themethod of least squares.

In this case, the luminous intensity before the elapse of 10⁻⁵ secondsis preferably fitted.

The TTF mechanism having an emission mechanism by delayed fluorescencewill be described using FIG. 7. FIG. 7 shows a relationship in energylevel between the host material and the dopant material in an organic ELdevice using the TTF mechanism. In FIG. 7, S0, S1 _(H), T1 _(H), S1 _(D)and T1 _(D) represent the same as those in FIG. 3. An arrow shows energytransfer between the respective excited states in FIG. 7.

As described above, the TTF mechanism utilizes a phenomenon in whichsinglet excitons are generated by collision between two tripletexcitons. As shown in FIG. 7, it is preferable that the lowest tripletstate T1 _(H) of the host material is lower than the lowest tripletstate T1 _(D) of the dopant material, so that triplet excitonsconcentrate on molecules of the host material. The triplet excitonsefficiently collide with each other in accordance with increase in thedensity of the triplet excitons, whereby the triplet excitons arepartially changed into singlet excitons. The lowest singlet state S1_(H) of the host material generated by the TTF mechanism is immediatelytransferred to the lowest singlet state S1 _(D) of the dopant materialby Førster transfer, so that the dopant material emits fluorescence.

The theoretical upper-limit of the TTF ratio can be obtained as follows.

According to S. M. Bachilo et al. (J. Phys. Chem. A, 104, 7711 (2000)),assuming that high-order excitons such as quintet excitons are quicklyreturned to triplet excitons, triplet excitons (hereinafter abbreviatedas ³A*) collide with one another when the density thereof is increased,whereby a reaction shown by the following numerical formula (15) occurs.In the formula, ¹A represents the ground state and ¹A* represents thelowest singlet excitons.

Numerical Formula 2

³ A*+ ³ A*→(4/9)¹ A+( 1/9)¹ A*+(13/9)³ A*  (15)

In other words, 5³A*→4¹A+¹A*. It is expected that, among tripletexcitons initially generated, which account for 75%, one fifth thereof(i.e., 20%) is changed to singlet excitons.

Accordingly, the amount of singlet excitons which contribute to emissionis 40%, which is a value obtained by adding 15% (75%×(⅕)=15%) to 25%(the amount ratio of initially generated singlet excitons).

At this time, a ratio of luminous intensity derived from TTF (TTF ratio)relative to the total luminous intensity is 15/40, i.e., 37.5%. Thus, itis recognized that the delayed fluorescence ratio of the organic ELdevice according to the exemplary embodiment exceeds the theoreticalupper-limit of only the TTF ratio.

Residual Intensity Ratio in 1 μs

A method for relatively measuring an amount of delayed fluorescence isexemplified by a method for measuring a residual intensity in 1 μs. Theresidual intensity in 1 μs is defined as a ratio of a luminous intensityafter the elapse of 1 μs after removal of a pulse voltage measured by atransitional EL method to a luminous intensity at the time of theremoval of the pulse voltage. The relative amount of delayedfluorescence can be estimated based on reduction behavior of EL emissionafter the removal of the pulse voltage measured by the transitional ELmethod. The residual intensity ratio in 1 μs can be obtained by readingluminous intensity at the time of 1.0 μs in the graph of FIG. 6A.

The residual intensity ratio in 1 μs is preferably larger than 36.0%,more preferably 38.0% or more.

ΔT

It is preferable that a difference ΔT between triplet energyEg_(77K)(H1) of the first host material and triplet energy Eg_(77K)(D)of the dopant material satisfies a relationship represented by thenumerical formula (7). ΔT is preferably more than 0.5 eV, morepreferably 0.8 eV or more, further preferably 1.0 eV or more.

When ΔT satisfies the relationship represented by the numerical formula(7), energy of the triplet excitons generated by recombination on thefirst host material becomes difficult to transfer to the triplet levelof the dopant material, and thermal deactivation of the triplet excitonsbecomes difficult. Consequently, the dopant efficiently emitsfluorescence.

Compound Used as First Host Material

In a combination of the second host material and the dopant material,the first host material satisfying the numerical formulae (1), (2) to(4) and (6) to (7) is preferably selected from the group consisting of acarbazole derivative, a biscarbazole derivative, an indolocarbazolederivative, an acridine derivative, an oxazine derivative, a pyrazinederivative, a pyrimidine derivative, a triazine derivative, adibenzofuran derivative, and a dibenzothiophene derivative. Thesederivatives may have a substituent as needed.

Examples of the substituent include an aryl group having 6 to 40 carbonatoms, a heterocyclic group having 2 to 40 carbon atoms, a trialkylsilylgroup, dialkylarylsilyl group, an alkyldiarylsilyl group, a triarylsilylgroup, a fluorine atom, and a cyano group. The trialkylsilyl group, thedialkylarylsilyl group, the alkyldiarylsilyl group, and the triarylsilylgroup as the substituent contain at least one of an alkyl group having 1to 30 carbon atoms and an aryl group having 6 to 30 carbon atoms. Itshould be noted that the aryl group in the substituent also includes afused aromatic hydrocarbon group and the heterocyclic group in thesubstituent also includes a fused aromatic heterocyclic group.

The host material is preferably a compound including bonding between atleast one selected from a carbazole structure, a biscarbazole structure,an indolocarbazole structure, and an acridine structure and at least oneselected from an oxazine structure, a pyrazine structure, a pyrimidinestructure, a triazine structure, and a dibenzofuran structure.

Bonding between these structures means bonding by various linkinggroups. Preferable examples of the linking group are a single bond, aphenylene structure and metabiphenylene structure.

In the exemplary embodiment, the carbazole structure, theindolocarbazole structure, the acridine structure, the oxadinestructure, the pyrazine structure, the pyrimidine structure, thetriazine structure, and the dibenzofuran structure respectively refer tocyclic structures containing indolocarbazole, acridine, oxadine,pyrazine, pyrimidine, triazine, and dibenzofuran as a partial structure.

The carbazole structure, the biscarbazole structure, the indolocarbazolestructure, the acridine structure, the oxazine structure, the pyrazinestructure, the pyrimidine structure, the triazine structure, and thedibenzofuran structure may have a substituent as needed.

Examples of the substituent include an aryl group having 6 to 40 carbonatoms, a heterocyclic group having 2 to 40 carbon atoms, a trialkylsilylgroup, dialkylarylsilyl group, an alkyldiarylsilyl group, a triarylsilylgroup, a fluorine atom, and a cyano group. The trialkylsilyl group, thedialkylarylsilyl group, the alkyldiarylsilyl group, and the triarylsilylgroup as the substituent contain at least one of an alkyl group having 1to 30 carbon atoms and an aryl group having 6 to 30 carbon atoms.

In the exemplary embodiment of the invention, a “hydrogen atom” meansisotopes having different neutron numbers and specifically encompassesprotium, deuterium and tritium.

Since the first host material is a compound in which a donor element isbonded to an acceptor element in a molecule, the first host material ispreferably a biscarbazole derivative represented by the followingformula (101).

In the above formula (101), A₁ and A₂ each independently represent ahydrogen atom, a halogen atom, a cyano group, a substituted orunsubstituted aromatic hydrocarbon group having 6 to 30 ring atoms, asubstituted or unsubstituted aromatic heterocyclic group having 2 to 30ring carbon atoms, a substituted or unsubstituted alkyl group having 1to 30 carbon atoms, a substituted or unsubstituted cycloalkyl grouphaving 3 to 30 ring carbon atoms, a substituted or unsubstituted alkoxygroup having 1 to 30 carbon atoms, a substituted or unsubstitutedaralkyl group having 7 to 30 carbon atoms, a substituted orunsubstituted aryloxy group having 6 to 30 ring carbon atoms, or asubstituted or unsubstituted silyl group.

However, it is preferable that at least one of A₁ and A₂ is a cyanogroup.

It should be noted that, in A₁ and A₂, the aromatic hydrocarbon groupincludes a fused aromatic hydrocarbon group and the aromaticheterocyclic group includes a fused aromatic heterocyclic group.

In the above formula (101), Y₁ to Y₄ and Y₁₃ to Y₁₆ independentlyrepresent C(R) or a nitrogen atom. Y₅ to Y₈ independently representC(R), a nitrogen atom or a carbon atom to be bonded to one of Y₉ to Y₁₂.Y₉ to Y₁₂ independently represent C(R), a nitrogen atom or a carbon atomto be bonded to one of Y₅ to Y₈. R independently represent a hydrogenatom or a substituent. The substituent in R is the same as thesubstituent described for the above first host material.

L₁ and L₂ in the above formula (101) each independently represent asingle bond, a substituted or unsubstituted divalent aromatichydrocarbon group having 6 to 30 ring carbon atoms, a substituted orunsubstituted divalent aromatic heterocyclic group having 2 to 30 ringatoms, or a group formed by bonding the above divalent aromatichydrocarbon group and the above divalent aromatic heterocyclic group.

It should be noted that, in L₁ and L₂, the aromatic hydrocarbon groupincludes a fused aromatic hydrocarbon group and the aromaticheterocyclic group includes a fused aromatic heterocyclic group.

At least one of L₁ and L₂ is preferably represented by a formula (a)below.

In the above formula (a), Y₂₁ to Y₂₅ each independently representC(R_(a)), a nitrogen atom or a carbon atom to be bonded to L₃, in whichR_(a) independently represents a hydrogen atom or a substituent. Thesubstituent in R_(a) is the same as the substituent described for theabove first host material.

L₃ and L₄ in the above formula (a) each independently represent a singlebond, a substituted or unsubstituted divalent aromatic hydrocarbon grouphaving 6 to 30 ring carbon atoms, a substituted or unsubstituteddivalent aromatic heterocyclic group having 2 to 30 ring atoms, or agroup formed by bonding the above divalent aromatic hydrocarbon groupand the above divalent aromatic heterocyclic group.

It should be noted that, in L₃ and L₄, the aromatic hydrocarbon groupincludes a fused aromatic hydrocarbon group and the aromaticheterocyclic group includes a fused aromatic heterocyclic group.

Second Host Material

In the exemplary embodiment, the second host material is a compoundhaving a fused aromatic hydrocarbon group having 10 to 30 ring carbonatoms or a fused aromatic heterocyclic group having 8 to 30 ring atoms.

The second host material is exemplified by an anthracene derivativerepresented by the following formula (20A), (20B), (20C), (20D) or(20E). However, the invention is not limited by the anthracenederivative having these structures.

In the formula (20A), R¹⁰¹ and R¹⁰⁵ each independently represent ahydrogen atom, halogen atom, cyano group, substituted or unsubstitutedmonocyclic group having 5 to 30 ring atoms, substituted or unsubstitutedfused cyclic group having 8 to 30 ring atoms, a group provided bycombining a monocyclic group and a fused cyclic group, substituted orunsubstituted alkyl group having 1 to 30 carbon atoms, substituted orunsubstituted cycloalkyl group having 3 to 30 ring carbon atoms,substituted or unsubstituted alkoxy group having 1 to 30 carbon atoms,substituted or unsubstituted aralkyl group having 7 to 30 carbon atoms,substituted or unsubstituted aryloxy group having 6 to 30 ring carbonatoms, or substituted or unsubstituted silyl group.

In the formula (20A), Ar⁵¹ and Ar⁵⁴ each independently represent asubstituted or unsubstituted divalent monocyclic residue having 5 to 30ring atoms, or substituted or unsubstituted divalent fused cyclicresidue having 8 to 30 ring atoms.

In the formula (20A), Ar⁵² and Ar⁵⁵ each independently represent asingle bond, substituted or unsubstituted divalent monocyclic residuehaving 5 to 30 ring atoms, or substituted or unsubstituted divalentfused cyclic residue having 8 to 30 ring atoms.

In the formula (20A), Ar⁵³ and Ar⁵⁶ each independently represent ahydrogen atom, substituted or unsubstituted monocyclic group having 5 to30 ring atoms, or substituted or unsubstituted fused cyclic group having8 to 30 ring atoms.

In the formula (20B), Ar⁵¹ represents a substituted or unsubstituteddivalent monocyclic residue having 5 to 30 ring atoms, or substituted orunsubstituted divalent fused cyclic residue having 8 to 30 ring atoms.

In the formula (20B), Ar⁵² and Ar⁵⁵ each independently represent asingle bond, substituted or unsubstituted divalent monocyclic residuehaving 5 to 30 ring atoms, or substituted or unsubstituted divalentfused cyclic residue having 8 to 30 ring atoms.

In the formula (20B), Ar⁵³ and Ar⁵⁶ each independently represent ahydrogen atom, substituted or unsubstituted monocyclic group having 5 to30 ring atoms, or substituted or unsubstituted fused cyclic group having8 to 30 ring atoms.

In the formula (20C), Ar⁵² represents a substituted or unsubstituteddivalent monocyclic residue having 5 to 30 ring atoms, or substituted orunsubstituted divalent fused cyclic residue having 8 to 30 ring atoms.

In the formula (20C), Ar⁵⁵ represents a single bond, substituted orunsubstituted divalent monocyclic residue having 5 to 30 ring atoms, orsubstituted or unsubstituted divalent fused cyclic residue having 8 to30 ring atoms.

In the formula (20C), Ar⁵³ and Ar⁵⁶ each independently represent ahydrogen atom, substituted or unsubstituted monocyclic group having 5 to30 ring atoms, or substituted or unsubstituted fused cyclic group having8 to 30 ring atoms.

In the formula (20D), Ar⁵² represents a substituted or unsubstituteddivalent monocyclic residue having 5 to 30 ring atoms, or substituted orunsubstituted divalent fused cyclic residue having 8 to 30 ring atoms.

In the formula (20D), Ar⁵⁵ represents a single bond, substituted orunsubstituted divalent monocyclic residue having 5 to 30 ring atoms, orsubstituted or unsubstituted divalent fused cyclic residue having 8 to30 ring atoms.

In the formula (20D), Ar⁵³ and Ar⁵⁶ each independently represent ahydrogen atom, substituted or unsubstituted monocyclic group having 5 to30 ring atoms, or substituted or unsubstituted fused cyclic group having8 to 30 ring atoms.

In the formula (20E), Ar⁵² and Ar⁵⁵ each independently represent asingle bond, substituted or unsubstituted divalent monocyclic residuehaving 5 to 30 ring atoms, or substituted or unsubstituted divalentfused cyclic residue having 8 to 30 ring atoms.

In the formula (20E), Ar⁵³ and Ar⁵⁶ each independently represent ahydrogen atom, substituted or unsubstituted monocyclic group having 5 to30 ring atoms, or substituted or unsubstituted fused cyclic group having8 to 30 ring atoms.

The monocyclic group in the formulae (20A), (20B), (20C), (20D) and(20E) refers to a group only having a cyclic structure without a fusedstructure.

The monocyclic group has 5 to 30 ring atoms, preferably 5 to 20 ringatoms. Examples of the monocyclic group are: an aromatic group such as aphenyl group, biphenyl group, terphenyl group, and quarterphenyl group;and a heterocyclic group such as a pyridyl group, pyrazyl group,pyrimidyl group, triazinyl group, furyl group, and thienyl group. Amongthe monocyclic group, a phenyl group, biphenyl group and terphenyl groupare preferable.

The fused cyclic group in the formulae (20A), (20B), (20C), (20D) and(20E) is a group obtained by fusing two or more cyclic structures.

The fused cyclic group has 8 to 30 ring atoms, preferably 8 to 20 ringatoms. Examples of the fused cyclic group are: a fused aromatic ringgroup such as a naphthyl group, phenanthryl group, anthryl group,chrysenyl group, benzanthryl group, benzophenanthryl group,triphenylenyl group, benzochrysenyl group, indenyl group, fluorenylgroup, 9,9-dimethylfluorenyl group, benzofluorenyl group,dibenzofluorenyl group, fluoranthenyl group, and benzofluoranthenylgroup; and a fused heterocyclic group such as a benzofuranyl group,benzothiophenyl group, indolyl group, dibenzofuranyl group,dibenzothiophenyl group, carbazolyl group, quinolyl group, andphenanthrolinyl group. Among the fused cyclic groups, a naphthyl group,phenanthryl group, anthryl group, 9,9-dimethylfluorenyl group,fluoranthenyl group, benzoanthryl group, dibenzothiophenyl group,dibenzofuranyl group and carbazolyl group are preferable.

Among the second host material to be combined with the first hostmaterial and the dopant material, a compound satisfying the abovenumerical formulae (2), (5) and (6) is preferable.

Dopant Material

A preferable dopant in the exemplary embodiment has properties to emitfluorescence and to have a large speed constant of radiationaltransition. In this arrangement, singlet excitons electrically excitedon the host material, singlet excitons generated by the TADF mechanismand the like are transferred to singlet excitons of the dopant materialby Førster energy transfer and the dopant material immediately emitslight. In other words, fluorescent emission is possible through theabove energy transition before triplet excitons on the host materialcauses TTA, by which decrease in an efficiency in the high currentdensity range is likely to be considerably improved.

It is preferable to select a dopant material having a fluorescencelifetime of 5 ns or less, more preferably 2 ns or less as the dopantmaterial having a large speed constant of radiational transition in theexemplary embodiment. A fluorescence quantum efficiency of the dopantmaterial is preferably 80% or more in a solution. The fluorescencequantum efficiency can be obtained by measuring the dopant material in arange of 10⁻⁵ to 10⁻⁶ mol/l of a concentration in a toluene solutionusing Absolute PL Quantum Yield Measurement System C9920-02 manufacturedby HAMAMATSU PHOTONICS K.K.

It is also expected by measuring an EL spectrum of the device andconfirming a luminescence component of a material other than the dopantmaterial is 1/10 or less of the luminescence component of the dopantthat the dopant material has a large speed constant of radiationaltransition.

Compound Used as Dopant Material

The dopant material may be a non-heavy-metal complex, i.e., afluorescent dopant material. Examples of the dopant material include anaphthalene derivative, an anthracene derivative, a pyrene derivative, achrysene derivative, a fluoranthene derivative, an indenoperylenederivative, a pyrromethene-boron complex compound, a compound having apyrromethene skeleton or a metal complex thereof, a diketopyrolopyrrolderivative, and a perylene derivative.

Examples of the naphthalene derivative include a bisarylaminonaphthalenederivative and an aryl-substituted naphthalene derivative. Examples ofthe anthracene derivative include a bisarylaminoanthracene derivativeand an aryl-substituted anthracene derivative. Examples of the pyrenederivative include a bisarylaminopyrene derivative and anaryl-substituted pyrene derivative. Examples of the chrysene derivativeinclude a bisarylaminochrysene derivative and an aryl-substitutedchrysene derivative.

The fluoranthene derivative is exemplified by compounds represented byformulae (2) to (18) below.

In the formulae (2) to (16), X¹ to X²⁰ each independently represent ahydrogen atom, a linear, branched or cyclic alkyl group having 1 to 20carbon atoms, a linear, branched or cyclic alkoxy group having 1 to 20carbon atoms, a substituted or unsubstituted aryl group having 6 to 30carbon atoms, a substituted or unsubstituted aryloxy group having 6 to30 carbon atoms, a substituted or unsubstituted arylamino group having 6to 30 carbon atoms, a substituted or unsubstituted alkylamino grouphaving 1 to 30 carbon atoms, a substituted or unsubstitutedarylalkylamino group having 7 to 30 carbon atoms, or a substituted orunsubstituted alkenyl group having 8 to 30 carbon atoms.

The adjacent substituents and X¹ to X²⁰ may be bonded together to form acyclic structure.

Further, the adjacent substituents, each of which is the aryl group, maybe the same with each other.

The compounds of formulae (2) to (16) each preferably contain an aminogroup or an alkenyl group, more preferably are represented by formulae(17) to (18) below.

In the formulae (17) and (18), X²¹ to X²⁴ each independently representan alkyl group having 1 to 20 carbon atoms or a substituted orunsubstituted aryl group having 6 to 30 carbon atoms.

X²¹ and X²² may be bonded by a carbon-carbon bond or through —O— and—S—.

X²³ and X²⁴ may be bonded by a carbon-carbon bond or through —O— and—S—.

X²⁵ to X³⁶ represent a hydrogen atom, a linear, branched or cyclic alkylgroup having 1 to 20 carbon atoms, a linear, branched or cyclic alkoxygroup having 1 to 20 carbon atoms, a substituted or unsubstituted arylgroup having 6 to 30 carbon atoms, a substituted or unsubstitutedaryloxy group having 6 to 30 carbon atoms, a substituted orunsubstituted arylamino group having 6 to 30 carbon atoms, a substitutedor unsubstituted alkylamino group having 1 to 30 carbon atoms, asubstituted or unsubstituted arylalkylamino group having 7 to 30 carbonatoms, or a substituted or unsubstituted alkenyl group having 8 to 30carbon atoms.

The adjacent substituents and X²⁵ to X³⁶ may be bonded together to forma cyclic structure.

At least one of the substituents X²⁵ to X³⁶ in the above formulae ispreferably amine or an alkenyl group.

A fluorescent compound having a fluoranthene skeleton preferablycontains an electron-donating group in order to achieve a highefficiency and a long lifetime. The electron-donating group ispreferably a substituted or unsubstituted acylamino group. Further, thefluorescent compound having a fluoranthene skeleton preferably has 5 ormore fused rings, particularly preferably 6 or more fused rings.

Relationship in Energy Level Among First Host Material, Second HostMaterial and Dopant Material

FIG. 8 shows a relationship in energy level among the first hostmaterial, the second and the dopant material in the emitting layer ofthe exemplary embodiment.

In FIG. 8: S0 represents a ground state; S1 _(H1) represents the lowestsinglet state of the first host material; T1 _(H1) represents the lowesttriplet state of the first host material; S1 _(H2) represents the lowestsinglet state of the second host material; T1 _(H2) represents thelowest triplet state of the second host material; S1 _(D) represents thelowest singlet state of the dopant material; and T1 _(D) represents thelowest triplet state of the dopant material. As shown in FIG. 3, adifference between S1 _(H1) and T1 _(H1) corresponds to ΔST(H1), adifference between S1 _(H1) and S0 corresponds to EgS(H1), a differencebetween S1 _(D) and S0 corresponds to EgS(D), and a difference betweenT1 _(H1) and T1 _(D1) corresponds to ΔT. In FIG. 8: a solid arrow showsenergy transfer between the states; a wavy arrow shows deactivation ofenergy with emission; and a dotted-line arrow shows deactivation ofenergy without emission.

As shown in FIG. 8, both of the TTF mechanism and the TADF mechanism areconsidered to exist in the emission mechanism of the exemplaryembodiment. In the first host material, a sufficiently small ΔST(H1)causes energy transfer by the TADF mechanism. Specifically, tripletexcitons decay through reverse intersystem crossing from the lowesttriplet state T1 _(H1) to the lowest singlet state S1 _(H1), in whichenergy is transferred to the lowest singlet state S1 _(D) of the dopantmaterial by Førster transfer. An internal quantum efficiency by thismechanism is theoretically 100%. In the second host material, energy istransferred using the TTF mechanism. Specifically, energy is transferredfrom the lowest singlet state S1 _(H2) to the lowest singlet state S1_(D) of the dopant material by Førster transfer, so that fluorescentemission can be obtained. An internal quantum efficiency by thismechanism is theoretically 40%.

Accordingly, in the emitting layer including the first host material andthe second host material in a mass ratio of 50:50, emission derived fromthe first host material amounts to 50% and emission derived from thesecond host material amounts to 20%, reaching totally theoretically 70%of the internal quantum efficiency.

Moreover, as shown in FIG. 8, when the lowest triplet state T1 _(H2) ofthe second host material is lower than the lowest triplet state T1 _(D)of the dopant material (i.e., the above numerical formula (5) issatisfied), energy is transferred from the lowest triplet state T1 _(D)to the lowest triplet state T1 _(H2), so that the TTF mechanism can bemore efficiently caused in the second host material.

Relationship Between Emitting Layer and Electron Transporting Layer

When ΔST(H1) of the first host material is small, the energy differencebetween the first host material and the electron transporting layeradjacent to the emitting layer is small, so that the electrons arelikely to be injected into the emitting layer. As a result, carrierbalance is easily obtainable to decrease roll-off.

Relationship Between Emitting Layer and Hole Transporting Layer

When an ionization potential of the hole transporting layer isrepresented by IP_(HT), IP_(HT)≤5.7 eV is preferable. With thisarrangement, balance between the electrons and the holes can beenhanced. The ionization potential can be obtained, for instance, bymeasuring the material in a form of a thin film using a photoelectronspectroscopy (AC-3: manufactured by RIKEN KEIKI Co., Ltd.).

Half Bandwidth

A half bandwidth represents a width of an emission spectrum when aluminous intensity becomes half relative to the maximum luminousintensity of the emission spectrum. The inventors found that a hostmaterial having 50 nm or more of a half bandwidth of a photoluminescencespectrum is a material easily forming an aggregate state and easilycausing inverse intersystem crossing in a thin film. Accordingly, theTADF mechanism easily works in the host material having 50 nm or more ofthe half bandwidth of the photoluminescence spectrum. Particularlypreferably, the half bandwidth of the photoluminescence spectrum of thehost material is 65 nm or more.

The thickness of the emitting layer is preferably in a range from 5 nmto 50 nm, more preferably from 7 nm to 50 nm, the most preferably from10 nm to 50 nm. The thickness of less than 5 nm may cause difficulty informing the emitting layer and in controlling chromaticity, while thethickness of more than 50 nm may raise drive voltage.

In the emitting layer, a ratio of the first host material to the secondhost material is preferably in a range of 80:20 to 20:80 at a massratio. Moreover, a ratio of the first and second host materials (i.e.,the host materials in total) to the fluorescent dopant material ispreferably in a range of 99:1 to 50:50 at a mass ratio.

Substrate

The organic EL device according to the exemplary embodiment is formed ona light-transmissive substrate. The light-transmissive substratesupports an anode, an organic compound layer, a cathode and the like ofthe organic EL device. The light-transmissive substrate is preferably asmoothly-shaped substrate that transmits 50% or more of light in avisible region of 400 nm to 700 nm.

The light-transmissive plate is exemplarily a glass plate, a polymerplate or the like.

The glass plate is formed of soda-lime glass,barium/strontium-containing glass, lead glass, aluminosilicate glass,borosilicate glass, barium borosilicate glass, quartz and the like.

The polymer plate is formed of polycarbonate, acryl, polyethyleneterephthalate, polyether sulfide, polysulfone and the like.

Anode and Cathode

The anode of the organic EL device injects holes into the emittinglayer, so that it is efficient that the anode has a work function of 4.5eV or more.

Exemplary materials for the anode are indium-tin oxide (ITO), tin oxide(NESA), indium zinc oxide, gold, silver, platinum and copper.

When light from the emitting layer is to be emitted through the anode,the anode preferably transmits more than 10% of the light in the visibleregion. Sheet resistance of the anode is preferably several hundredsΩ/square or lower. Although depending on the material of the anode, thethickness of the anode is typically in a range of 10 nm to 1 μm,preferably in a range of 10 nm to 200 nm.

The cathode is preferably formed of a material with smaller workfunction in order to inject electrons into the emitting layer.

Although a material for the cathode is subject to no specificlimitation, examples of the material are indium, aluminum, magnesium,alloy of magnesium and indium, alloy of magnesium and aluminum, alloy ofaluminum and lithium, alloy of aluminum, scandium and lithium, and alloyof magnesium and silver.

Like the anode, the cathode may be made by forming a thin film on, forinstance, the electron transporting layer and the electron injectinglayer by a method such as vapor deposition. In addition, the light fromthe emitting layer may be emitted through the cathode. When light fromthe emitting layer is to be emitted through the cathode, the cathodepreferably transmits more than 10% of the light in the visible region.

Sheet resistance of the cathode is preferably several hundreds Ω persquare or lower.

The thickness of the cathode is typically in the range of 10 nm to 1 E,and preferably in the range of 50 nm to 200 nm, though it depends on thematerial of the cathode.

Hole Injecting/Transporting Layer

The hole injection/transport layer helps injection of holes to theemitting layer and transport the holes to an emitting region. A compoundhaving a large hole mobility and a small ionization energy is used asthe hole injection/transport layer.

A material for forming the hole injection/transport layer is preferablya material of transporting the holes to the emitting layer at a lowerelectric field intensity. For instance, an aromatic amine compound ispreferably used.

Electron Injecting/Transporting Layer

The electron injecting/transporting layer helps injection of theelectrons into the emitting layer and transports the electrons to anemitting region. A compound having a large electron mobility is used asthe electron injecting/transporting layer.

A preferable example of the compound used as the electroninjecting/transporting layer is an aromatic heterocyclic compound havingat least one heteroatom in a molecule. Particularly, anitrogen-containing cyclic derivative is preferable. Thenitrogen-containing cyclic derivative is preferably a heterocycliccompound having a nitrogen-containing six-membered or five-membered ringskeleton.

In the organic EL device in the exemplary embodiment, in addition to theabove exemplary compounds, any compound selected from compounds used ina typical organic El device is usable as a compound for the organiccompound layer other than the emitting layer.

Layer Formation Method(s)

A method for forming each layer of the organic EL device in theexemplary embodiment is subject to no limitation except for the aboveparticular description.

However, known methods of dry film-forming such as vacuum deposition,sputtering, plasma or ion plating and wet film-forming such as spincoating, dipping, flow coating or ink-jet are applicable.

Modification(s) of Embodiment(s)

It should be noted that the invention is not limited to the aboveexemplary embodiment but may include any modification and improvement aslong as such modification and improvement are compatible with theinvention.

The emitting layer is not limited to a single layer, but may be providedas laminate by a plurality of emitting layers. When the organic ELdevice includes the plurality of emitting layers, it is only requiredthat at least one of the emitting layers includes the first hostmaterial, the second host material and the fluorescent dopant materialdefined in the exemplary embodiment. The others of the emitting layersmay be either a fluorescent emitting layer or a phosphorescent emittinglayer.

When the organic EL device includes the plurality of emitting layers,the plurality of emitting layers may be adjacent to each other.

Further, the materials and treatments for practicing the invention maybe altered to other arrangements and treatments as long as such otherarrangements and treatments are compatible with the invention.

EXAMPLES

Examples of the invention will be described below. However, theinvention is not limited by these Examples.

The used compounds were as follows:

Evaluation of Compounds

Next, properties of the compounds used in Examples were measured. Ameasurement method and a calculation method are described below.Measurement results and calculation results are shown in Table 1.

(Measurement 1) Singlet Energy EgS

Singlet Energy EgS was obtained by the following method.

A target compound to be measured was deposited on a quartz substrate toprepare a sample. An absorption spectrum of the sample was measured at anormal temperature (300K). A sample was 100 nm thick. The absorptionspectrum was expressed in coordinates of which ordinate axis indicatedabsorbance and of which abscissa axis indicated the wavelength. Atangent was drawn to the fall of the absorption spectrum on thelong-wavelength side, and a wavelength value λ edge (nm) at anintersection of the tangent and the abscissa axis was obtained. Thewavelength value was converted to an energy value by the followingconversion equation. The energy value was defined as EgS.

The conversion equation: EgS (eV)=1239.85/λ edge

For the measurement of the absorption spectrum, a spectrophotometer(U3310 manufactured by Hitachi, Ltd.) was used.

The tangent to the fall of the absorption spectrum on thelong-wavelength side was drawn as follows. While moving on a curve ofthe absorption spectrum from the maximum spectral value closest to thelong-wavelength side in a long-wavelength direction, a tangent at eachpoint on the curve is checked. An inclination of the tangent isdecreased and increased in a repeated manner as the curve falls (i.e., avalue of the ordinate axis is decreased). A tangent drawn at a point ofthe minimum inclination closest to the long-wavelength side (except whenabsorbance is 0.1 or less) is defined as the tangent to the fall of theabsorption spectrum on the long-wavelength side.

The maximum absorbance of 0.2 or less was not included in theabove-mentioned maximum absorbance on the long-wavelength side.

(Measurement 2) Energy Gap Eg_(77K) and Triplet Energy EgT_(D)

Eg_(77K) and EgT_(D) were obtained by the following method.

Each of the compounds was measured by a known method of measuringphosphorescence (e.g. a method described in “Hikarikagaku no Sekai (TheWorld of Photochemistry)” (edited by The Chemical Society of Japan,1993, on and near page 50). Specifically, each of the compounds wasdissolved in a solvent (sample: 10 μol/L, EPA(diethylether:isopentane:ethanol=5:5:2 in volume ratio, each solvent ina spectroscopic grade), thereby forming a sample for phosphorescencemeasurement. The sample for phosphorescence measurement was put into aquartz cell, cooled to 77K and irradiated with excitation light, so thatphosphorescence intensity was measured while changing a wavelength. Thephosphorescence spectrum was expressed in coordinates of which ordinateaxis indicated phosphorescence intensity and of which abscissa axisindicated the wavelength.

A tangent was drawn to the rise of the phosphorescent spectrum on theshort-wavelength side, and a wavelength value λ edge (nm) at anintersection of the tangent and the abscissa axis was obtained. Thewavelength value was converted to an energy value by the followingconversion equation. The energy value was defined as Eg_(77K)(H) orEgT_(D) (Eg_(77K)(D)).

The conversion equation: Eg _(77K)(H) (eV)=1239.85/λ edge

:EgT _(D) [eV]=1239.85/λ edge

The tangent to the rise of the phosphorescence spectrum on theshort-wavelength side was drawn as follows. While moving on a curve ofthe phosphorescence spectrum from the short-wavelength side to themaximum spectral value closest to the short-wavelength side among themaximum spectral values, a tangent is checked at each point on the curvetoward the long-wavelength of the phosphorescence spectrum. Aninclination of the tangent is increased as the curve rises (i.e., avalue of the ordinate axis is increased). A tangent drawn at a point ofthe maximum inclination was defined as the tangent to the rise of thephosphorescence spectrum on the short-wavelength side.

The maximum with peak intensity being 10% or less of the maximum peakintensity of the spectrum is not included in the above-mentioned maximumclosest to the short-wavelength side of the spectrum. The tangent drawnat a point of the maximum spectral value being closest to theshort-wavelength side and having the maximum inclination is defined as atangent to the rise of the phosphorescence spectrum on theshort-wavelength side.

For phosphorescence measurement, a spectrophotofluorometer body F-4500and optional accessories for low temperature measurement (which weremanufactured by Hitachi High-Technologies Corporation) were used. Themeasurement instrument is not limited to this arrangement. A combinationof a cooling unit, a low temperature container, an excitation lightsource and a light-receiving unit may be used for measurement.

(Measurement 3) ΔST

ΔST was obtained as a difference between EgS and Eg_(77K) measured inthe above Measurement 1 and Measurement 2 (see the above numericalformula (3)). The results are shown in Table 1.

A half bandwidth of photoluminescence spectrum was obtained as follows.

Each of the compounds was formed in a 100 nm-thick film on a glasssubstrate with a deposition apparatus to prepare a sample forfluorescence measurement.

The sample for fluorescence measurement was placed so as to beirradiated with light in a direction orthogonal to the glass substrate.The sample for fluorescence measurement was irradiated with excitationlight at a room temperature 300K, so that fluorescence intensity wasmeasured while changing a wavelength.

The photoluminescence spectrum was expressed in coordinates of whichordinate axis indicated fluorescence intensity and of which abscissaaxis indicated the wavelength. For fluorescence measurement, aspectrophotofluorometer F-4500 (manufactured by HitachiHigh-Technologies Corporation) was used.

The half bandwidth (unit: nm) was measured based on thephotoluminescence spectrum.

The compounds H1-1 and H1-2 were measured in terms of the halfbandwidth. The results are shown in Table 1.

TABLE 1 EgS (thin film) Eg(77K) ΔST Half bandwidth [eV] [eV] [eV] [nm]H1-1 2.99 2.71 0.28 67 H1-2 3.02 2.74 0.28 63

Preparation and Evaluation of Organic EL Device

The organic EL devices were prepared in the following manner andevaluated.

Example 1

A glass substrate (size: 25 mm×75 mm×1.1 mm thick, manufactured byGeomatec Co., Ltd.) having an ITO transparent electrode (anode) wasultrasonic-cleaned in isopropyl alcohol for five minutes, and thenUV/ozone-cleaned for 30 minutes. A film of ITO was 77 nm thick.

After the glass substrate having the transparent electrode line wascleaned, the glass substrate was mounted on a substrate holder of avacuum evaporation apparatus. Initially, a compound HA-1 was depositedon a surface of the glass substrate where the transparent electrode linewas provided in a manner to cover the transparent electrode, therebyforming a 5-nm thick film of the compound HA-1. The HA-1 film serves asa hole injecting layer.

After the film formation of the HA-1 film, a compound HT-1 was depositedon the HA-1 film to form a 65-nm thick HT-1 film. The HT-1 film servesas a first hole transporting layer.

A compound HT-2 was deposited on the HT-1 film to form a 10-nm thickHT-2 film. The HT-2 film serves as a second hole transporting layer.

Further, a compound H1-1 (the first host material), a compound H2-1 (thesecond host material) and a compound YD-1 (the dopant material) wereco-deposited on the HT-2 film to form a 25-nm thick emitting layer. Aconcentration of the first host material was set at 40 mass %, aconcentration of the second host material was set at 40 mass %, and aconcentration of the dopant material was set at 20 mass % in theemitting layer.

An electron transporting compound ET-1 was deposited on the emittinglayer to form a 5-nm thick hole blocking layer.

Further, a compound ET-2 was deposited on the ET-1 film to form a 30-nmthick electron transporting layer.

LiF was deposited on the electron transporting layer to form a 1-nmthick LiF layer.

A metal Al was deposited on the LiF film to form an 80-nm thick metalcathode.

A device arrangement of the organic EL device in Example 1 isschematically shown in Table 2.

In Table 2, numerals in parentheses represent a film thickness (unit:nm). Likewise, the numerals represented by percentage in parentheses,which is positioned after the numerals representing the film thickness,indicate a material concentration in the emitting layer. In the emittinglayer including the first and second host materials, a massconcentration (unit: mass %) of the second host material and a massconcentration (unit: mass %) of the dopant material are shown in thisorder. When the emitting layer includes only one of the host materials,a mass concentration of the dopant material in the emitting layer isshown. Further, in the layers other than the emitting layer, a massconcentration (unit: mass %) of an added component other than the maincomponent is shown.

A material for forming the emitting layer is separately shown in Table3.

Comparatives 1 to 2

In Comparatives 1 to 2, organic EL devices were prepared in the samemanner as in the Example 1 so as to each have a device arrangement asshown in Table 2 by replacing the materials for the emitting layer.

TABLE 2 Device Arrangement Example 1ITO(77)/HA-1(5)/HT-1(65)/HT-2(10)/H1-1:H2-1:YD-1(25:40%,20%)/ET-1(5)/ET-2(30)/LiF(1)/Al(80) Comparative 1ITO(77)/HA-1(5)/HT-1(65)/HT-2(10)/H1-1:YD-1(25:20%)/ET-1(5)/ET-2(30)/LiF(1)/Al(80)Comparative 2ITO(77)/HA-1(5)/HT-1(65)/HT-2(10)/H2-1:YD-1(25:20%)/ET-1(5)/ET-2(30)/LiF(1)/Al(80)

TABLE 3 First Host Second Host Dopant Material Material Material Example1 H1-1 112-1 YD-1 Comparative 1 H1-1 None YD-1 Comparative 2 None H2-1YD-1

Evaluation of Organic EL Devices

The prepared organic EL devices were evaluated in terms of drivevoltage, luminous intensity, CIE1931 chromaticity, current efficiencyL/J, power efficiency η, main peak wavelength λ_(p), external quantumefficiency EQE, and a delayed fluorescence ratio. Evaluation results ofthe above points except for the delayed fluorescence ratio at thecurrent density of 1.00 mA/cm² are shown in Table 4.

Drive Voltage

Electrical current was applied between ITO and Al such that a currentdensity was 1.00 mA/cm², where voltage (unit: V) was measured.

CIE1931 Chromaticity

Voltage was applied on each of the organic EL devices such that acurrent density was 1.00 mA/cm², where coordinates(x, y) of CIE1931chromaticity were measured by a spectroradiometer (CS-1000 manufacturedby Konica Minolta Holdings, Inc.).

Current Efficiency L/J and Power Efficiency η

Voltage was applied on each of the organic EL devices such that thecurrent density was 1.00 mA/cm², where spectral radiance spectra weremeasured by the aforementioned spectroradiometer. Based on the obtainedspectral radiance spectra, the current efficiency (unit: cd/A) and thepower efficiency η (unit: 1 m/W) were calculated.

Main Peak Wavelength λ_(p)

A main peak wavelength λ_(p) was calculated based on the obtainedspectral-radiance spectra.

External Quantum Efficiency EQE

The external quantum efficiency EQE (unit: %) was calculated based onthe obtained spectral-radiance spectra, assuming that the spectra wasprovided under a Lambertian radiation.

Delayed Fluorescence Ratio

Voltage pulse waveform (pulse width: 500 micro second, frequency: 20 Hz,voltage: equivalent to 0.1 to 100 mA/cm²) output from a pulse generator(8114A: manufactured by Agilent Technologies) was applied. EL emissionwas input in a photomultiplier (R928: manufactured by HamamatsuPhotonics K.K.). The pulse voltage waveform and the EL emission weresynchronized and loaded in an oscilloscope (2440: manufactured byTektronix Company) to obtain a transitional EL waveform. Reciprocalnumbers of square root of luminous intensity were plotted, which werefitted in a linear line using a value before the elapse of 10⁻⁵ secondscalculated by the method of least squares to determine a delayedfluorescence ratio.

The transitional EL waveform where voltage of 1.00 mA/cm² was applied onthe organic EL device of the Example 1 at the room temperature is shownin FIG. 9. The pulse voltage was removed at the time of about 3×10⁻⁸seconds.

Based on the graph, where the voltage removal time was a starting pointand the reciprocal numbers of the square root of luminous intensitybefore the elapse of 1.5×10⁻⁵ seconds after voltage removal wereplotted, the delayed fluorescence ratio of the organic EL device of theExample 1 was 49.0%. This delayed fluorescence ratio exceeded thetheoretical upper-limit (37.5%) of the TTF ratio.

It was read from the graph in FIG. 9 that a residual intensity ratio in1 μs was 50.0%.

TABLE 4 Voltage chroma- chroma- L/J η [V] at 1 ticity ticity [cd/ [lm/EQE λp mA/cm² x y A] W] [%] [nm] Example 1 3.41 0.525 0.473 30.69 28.2710.81 575 Compara- 3.37 0.539 0.459 14.79 13.77 5.49 580 tive 1 Compara-3.10 0.513 0.485 27.52 27.93 9.29 571 tive 2

Example 2

A glass substrate (size: 25 mm×75 mm×1.1 mm thick, manufactured byGeomatec Co., Ltd.) having an ITO transparent electrode (anode) wasultrasonic-cleaned in isopropyl alcohol for five minutes, and thenUV/ozone-cleaned for 30 minutes. A film of ITO was 77 nm thick.

After the glass substrate having the transparent electrode line wascleaned, the glass substrate was mounted on a substrate holder of avacuum evaporation apparatus. Initially, a compound HA-1 was depositedon a surface of the glass substrate where the transparent electrode linewas provided in a manner to cover the transparent electrode, therebyforming a 5-nm thick film of the compound HA-1. The HA-1 film serves asa hole injecting layer.

After the film formation of the HA-1 film, a compound HT-1 was depositedon the HA-1 film to form a 125-nm thick HT-1 film. The HT-1 film servesas a first hole transporting layer.

A compound HT-2 was deposited on the HT-1 film to form a 25-nm thickHT-2 film. The HT-2 film serves as a second hole transporting layer.

Further, a compound H1-2 (the first host material), a compound H2-2 (thesecond host material) and a compound BD-1 (the dopant material) wereco-deposited on the HT-2 film to form a 25-nm thick emitting layer. Aconcentration of the first host material was set at 48 mass %, aconcentration of the second host material was set at 48 mass %, and aconcentration of the dopant material was set at 4 mass % in the emittinglayer.

An electron transporting compound ET-3 was deposited on the emittinglayer to form a 5-nm thick hole blocking layer.

Further, the compound ET-2 and Liq were co-deposited on the ET-3 film toform a 20-nm thick electron transporting layer. The concentration of Liqwas set at 50 mass %.

Liq was deposited on the electron transporting layer to form a 1-nmthick Liq film.

A metal Al was deposited on the Liq film to form an 80-nm thick metalcathode.

A device arrangement of each of the organic EL devices in Example 2 andComparative 3 is schematically shown in Table 5.

A material for forming the emitting layer is separately shown in Table6.

The organic EL device in Example 2 was evaluated in the same manner asin Example 1 in terms of drive voltage, luminous intensity, CIE1931chromaticity, current efficiency L/J, power efficiency η, main peakwavelength λ_(p), external quantum efficiency EQE, and a delayedfluorescence ratio. Evaluation results of the above items except for thedelayed fluorescence ratio at the current density of 1.00 mA/cm² areshown in Table 7 and those at the current density of 10.00 mA/cm² areshown in Table 8.

The transitional EL waveform where voltage of electrical current wasapplied at 1.00 mA/cm² on the organic EL device of the Example 2 at theroom temperature is shown in FIG. 10. The delayed fluorescence ratio ofthe organic EL device of the Example 2 calculated in the same manner asthe above based on the transitional EL waveform shown in FIG. 10 was43.0%.

It was read from the graph in FIG. 10 that a residual intensity ratio in1 μs was 47.0%.

Comparative 3

An organic EL device in Comparative 3 was prepared in the same manner asin the Example 2 so as to have a device arrangement as shown in Table 5by replacing the materials for the emitting layer.

The organic EL device prepared in Comparative 3 was evaluated in thesame manner as in Example 1. Evaluation results of the above pointsexcept for the delayed fluorescence ratio at the current density of 1.00mA/cm² are shown in Table 7 and those at the current density of 10.00mA/cm² are shown in Table 8.

With respect to the organic EL device in each of Examples 1 to 2 andComparatives 1 to 3, luminous efficiency (L/J) was standardized andplotted so as to show a relationship between the current density(mA/cm²) and the luminous efficiency. FIG. 11 shows the luminousefficiency in Example 1 and Comparatives 1 and 2. FIG. 12 shows theluminous efficiency in Example 2 and Comparative 3. In FIGS. 11 and 12,an abscissa axis shows the current density (mA/cm²) while an ordinateaxis shows a value obtained by standardizing the luminous efficiency(L/J arbitrary unit(a.u.)). Herein, with respect to the organic ELdevice in Example 1 in FIG. 11 and the organic EL device in Example 2 inFIG. 12, the value obtained by standardizing the luminous efficiency isa value calculated with the proviso that the maximum luminous efficiencyin each of Examples is defined as 1.

TABLE 4 Device Arrangement Example 2ITO(77)/HA-1(5)/HT-1(125)/HT-2(25)/H1-2:H2-2:BD-1(25:48%,4%)/ET-3(5)/ET-2:Liq(20,50%)/ Liq(1)/Al(80) Comparative 3ITO(77)/HA-1(5)/HT-1(125)/HT-2(25)/H1-2:BD-1(25:4%)/ET-3(5)/ET-2:Liq(20,50%)/Liq(1)/Al(80)

TABLE 5 First Host Second Host Dopant Material Material Material Example2 H1-2 H2-2 BD-1 Comparative 3 H1-2 None BD-1

TABLE 6 Voltage chroma- chroma- L/J η [V] at 1 ticity ticity [cd/ [lm/EQE λp mA/cm² x y A] W] [%] [nm] Example 2 3.45 0.128 0.208 11.12 10.147.75 472 Compara- 3.54 0.132 0.205 6.92 6.14 4.84 472 tive 3

TABLE 7 Voltage chroma- chroma- L/J η [V] at 10 ticity ticity [cd/ [lm/EQE λp mA/cm² x y A] W] [%] [nm] Example 2 4.28 0.128 0.205 9.92 7.296.97 472 Compara- 4.52 0.132 0.195 5.74 3.99 4.15 472 tive 3

As obviously understood from Tables 4, 7 and 8 and FIGS. 11 and 12, theorganic EL devices of Examples 1 and 2 emitted light with a highefficiency in a practically high current density range by using thefirst and second host materials satisfying the aforementionedrelationship in the emitting layer, as compared with Comparatives 1 and3 in which only the first host material is used (see Table 3, FIG. 11;Table 6, FIG. 12) and Comparative 2 in which only the second hostmaterial is used (see Table 6, FIG. 12).

REFERENCE EXAMPLE

Herein, the organic EL device described in the non-Patent Literature 1is shown as a reference example and compared with the organic EL deviceof Example 1 in terms of the device arrangement.

A device arrangement of the organic EL devices in the reference exampleis schematically shown below in the same manner as in Example 1.

ITO(110)/NPD(40)/m-CP(10)/m-CP:PIC-TRZ(20.6%)/BP4mPy(40)/LiF(0.8)/Al(70)Compounds used in the reference example will be shown below.

The device only exhibits the maximum EQE of 5.1% in the current densityrange of 0.01 mA/cm² which is much lower than the current density rangein a practical use. Accordingly, in a high current density range around1 mA/cm² and 10 mA/cm², roll-off is generated to reduce a luminousefficiency.

Accordingly, it is recognized that the organic EL devices of Examples 1and 2 emitted light with a high efficiency even in the high currentdensity range.

INDUSTRIAL APPLICABILITY

The invention can provide an organic electroluminescence deviceefficiently emitting light in a practically high current density range.

EXPLANATION OF CODE(S)

1: organic EL device, 2: substrate, 3: anode, 4: cathode, 5: emittinglayer, 6: hole transporting layer, 7: electron transporting layer

1. An organic electroluminescence device comprising a pair of electrodesand an organic compound layer therebetween, the organic compound layercomprising an emitting layer comprising: a first material; a secondmaterial; and a third material, wherein singlet energy EgS(H1) of thefirst material, singlet energy EgS(H2) of the second material, andsinglet energy EgS(D) of the third material satisfy a relationship ofnumerical formulae (1) and (2) below, a difference ΔST(H1) between thesinglet energy EgS(H1) of the first material and an energy gapEg_(77K)(H1) at 77K of the first material satisfies a relationship of anumerical formula (3) below, the second host material is a compoundhaving a fused aromatic hydrocarbon group having 10 to 30 ring carbonatoms or a fused aromatic heterocyclic group having 8 to 30 ring atoms,and the third material is a fluorescent material,EgS(H1)>EgS(D)  (1)EgS(H2)>EgS(D)  (2)ΔST(H1)=EgS(H1)−Eg _(77K)(H1)<0.3 [eV]  (3).
 2. The organicelectroluminescence device according to claim 1, wherein the differenceΔST(H1) between the singlet energy EgS(H1) of the first material and theenergy gap Eg_(77K)(H1) at 77K of the first material satisfies arelationship of a numerical formula (4) below,ΔST(H1)=EgS(H1)−Eg _(77K)(H1)<0.2 [eV]  (4).
 3. The organicelectroluminescence device according to claim 1, wherein an energy gapEg_(77K)(H2) at 77K of the second host material and an energy gapEg_(77K)(D) at 77K of the third material satisfy a relationship of anumerical formula (5) below,Eg _(77K)(H2)<Eg _(77K)(D)  (5)
 4. The organic electroluminescencedevice according to claim 1, wherein the energy gap Eg_(77K)(H1) at 77Kof the first host material and the energy gap Eg_(77K)(H2) at 77K of thesecond material satisfy a relationship of a numerical formula (6) below,Eg _(77K)(H1)−Eg _(77K)(H2)>0.5 [eV]  (6).
 5. The organicelectroluminescence device according to claim 1, wherein the energy gapEg_(77K)(H1) at 77K of the first host material and the energy gapEg_(77K)(D) at 77K of the third material satisfy a relationship of anumerical formula (7) below,Eg _(77K)(H1)−Eg _(77K)(D)>0.5 [eV]  (7).
 6. The organicelectroluminescence device according to claim 1, wherein the organicelectroluminescence device exhibits a delayed fluorescence ratio largerthan 37.5%.
 7. The organic electroluminescence device according to claim1, wherein the organic electroluminescence device exhibits a residualintensity ratio larger than 36.0% after an elapse of 1 μs after voltageremoval in a transitional EL measurement.
 8. The organicelectroluminescence device according to claim 1, wherein a halfbandwidth of a photoluminescence spectrum of the first material is 50 nmor more.
 9. The organic electroluminescence device according to claim 1,wherein an emission peak wavelength of the third material is in a rangeof 500 nm to 600 nm.
 10. An organic electroluminescence devicecomprising a pair of electrodes and an organic compound layertherebetween, the organic compound layer comprising an emitting layercomprising: a first material; a second material; and a third material,wherein singlet energy EgS(H1) of the first material, singlet energyEgS(H2) of the second material, and singlet energy EgS(D) of the thirdmaterial satisfy a relationship of numerical formulae (1) and (2) below,a difference ΔST(H1) between the singlet energy EgS(H1) of the firstmaterial and an energy gap Eg_(77K)(H1) at 77K of the first materialsatisfies a relationship of a numerical formula (3) below, the energygap Eg_(77K)(H1) at 77K of the first host material and an energy gapEg_(77K)(H2) at 77K of the second material satisfy a relationship of anumerical formula (6) below, and the third material is a fluorescentmaterial,EgS(H1)>EgS(D)  (1)EgS(H2)>EgS(D)  (2)ΔST(H1)=EgS(H1)−Eg _(77K)(H1)<0.3 [eV]  (3)Eg _(77K)(H1)−Eg _(77K)(H2)>0.5 [eV]  (6).